1. Field of the Invention
The present invention relates to linear accelerators, electrical switches and more particularly to very high-voltage and high-current switches, such as are needed for dielectric-wall linear accelerators and pulse-forming lines that operate at high gradients, e.g., in excess of twenty megavolts per meter.
2. Description of Related Art
Donald W. Hunter describes a laser-initiated dielectric-breakdown switch in U.S. Pat. No. 5,249,095, issued Sep. 28, 1993. Such switches are used in safe and arm systems for initiating exploding foil initiators. One electrode has an opening which allows light from a laser source to shine on dielectric material to induce voltage breakdown. Electrical conduction is precipitated through a dielectric, by solid dielectric breakdown between the electrodes, and this switch closing allows energy to pass from a power supply to the electronic foil initiator (EFI). Switches with high voltage ratings, e.g., tens of thousands of volts, are needed to hold off the magnitude of voltages typically found on an energy storage capacitor, e.g., 2-3 kilovolts (kV), for a single EFI. When triggered, such switches must produce an unusually fast rise time pulse, in order to initiate the EFI. Typical pulses must have stored energies of 0.3-0.6 millijoules, rise times of 30-60 nanoseconds, peak currents of 3-7 kiloamps (kA), and peak powers of 5-15 megawatts (MW). A commonly used switch for such applications is the ceramic body, hard brazed, miniature spark gap, with either an internal vacuum or a gas filled volume. But such spark gaps require hermetic sealing, are expensive, have marginal reliability and operating life, and require an expensive high voltage trigger circuit. One other switch in use for this application is the explosively initiated shock conduction switch which uses a primary explosive detonator. But this presents handling problems and can produce chemical contamination and possible explosive damage to surrounding electronics.
Other, conventional types of miniature switches include embedded electrode dielectric breakdown switches, e.g., as marketed by Mound Labs MLM-MC-88-28-000, reverse-bias diode avalanche switches, e.g., as marketed by Quantic Industries and Mound Labs, that are either electrically or light initiated, and gallium arsenide bulk conduction switches. But embedded electrode dielectric breakdown switches require a high voltage and a relatively high-energy trigger pulse from an expensive trigger circuit. Reverse bias diode avalanche switches require a significant number of components for both the switch and trigger circuit. Gallium arsenide switches are expensive, may require hermetic sealing, and often require high power for initiation, e.g., much more power than a laser diode can provide.
Particle accelerators are used to increase the energy of electrically-charged atomic particles, e.g., electrons, protons, or charged atomic nuclei, so that they can be studied by nuclear and particle physicists. High energy electrically-charged atomic particles are accelerated to collide with target atoms, and the resulting products are observed with a detector. At very high energies the charged particles can break up the nuclei of the target atoms and interact with other particles. Transformations are produced that help to discern the nature and behavior of fundamental units of matter. Particle accelerators are also important tools in the effort to develop nuclear fusion devices.
The energy of a charged particle is measured in electron volts, where one electron volt is the energy gained by an electron when it passes between electrodes having a potential difference of one volt. A charged particle can be accelerated by an electric field toward a charge opposite that of the charged particle. Beams of particles can be magnetically focused, and superconducting magnets can be used to advantage. Early machines in nuclear physics used static, or direct, electric fields. Most modern machines, particularly those for the highest particle energies, use alternating fields, where particles are exposed to the field only when the field is in the accelerating direction. When the field is reversed in the decelerating direction, the particles are shielded from the field by various electrode configurations.
The simplest radio frequency accelerator is the linear accelerator, or linac, and comes in different forms, depending electrons or ions are to be accelerated. For accelerating ions, frequencies of under 200 MHz are used. The ions are injected along the axis of a long tank excited by high-power radio frequency in an electric field along the axis. The ions are shielded from the decelerating phases by drift tubes in the tank through which the beam passes. As the particles gain energy and velocity, they travel farther. Therefore, the drift tubes must be longer toward the end of the tank to match the period of the accelerating field.
The first linear accelerator had three drift tubes and was built in 1928 by Rolf Wideroe of Norway. Sodium and potassium ions were accelerated to demonstrate the principle of radio frequency acceleration. During the 1930's, the University of California did further work on ion-type linear accelerators. But application of the principle was delayed until after World War II because of a lack of high-power radio frequency amplifiers. The development of radar provided such amplifiers. Shortly after the war, Luis Walter Alvarez built the first proton linear accelerator in which protons reached an energy of 32 million electron volts (MeV). Two megawatts were required at a frequency of about 200 MHz and limited the machine to one millisecond pulses.
Since 1950, several proton and ion linear accelerators have been built, some as injectors for still larger machines and some for use in nuclear physics. A large modern accelerator is the 800-MeV machine at the Los Alamos Scientific Laboratory, New Mexico, and is used as a meson factory in the study of intermediate-mass particles, e.g., those with masses heavier than the electron and lighter than the proton. These intermediate-mass particles seem to provide the force that binds atomic nucleus.
Because electrons are much lighter than ions, their velocity at a given energy is significantly higher than that of ions. The velocity of a one-MeV proton is less than five percent that of light. In contrast, a one-MeV electron has reached ninety-four percent of the velocity of light. This makes it possible to operate electron linacs at much higher frequencies, e.g., about 3,000 MHz. The accelerating system for electrons can be a few centimeters in diameter. The accelerating systems for ions need diameters of a few meters. Electron linacs having energies of ten to fifty MeV are widely used as x-ray sources for treating tumors with intense radiation.
A very large electron linac, which began operation in 1966 at the Stanford Linear Accelerator Center (California), is more than 3.2 km (2 mi.) long and has been able to provide electrons with energies of fifty billion electron volts (50 GeV). The Stanford Linear Collider can provide relative collisions that produce energies of more than 100 GeV between a beam of electrons and a beam of positrons that are aimed to collide head-on.
Such conventional accelerators are primarily useful for low currents, due to the interaction of the beam with the accelerator structure and the applied electric field. Induction accelerator types avoid many such problems.
FIG. 1 shows a cross-section of a single induction accelerator cell in which an accelerating voltage appears only across an internal accelerating gap. The cell housing and the outside of the accelerator are at ground potential. A large number of induction cells can be stacked in series to produce high energy beams without needing proportionately high voltages outside the accelerator that can be dangerous and troublesome to maintain. The core is a solid cylinder of either ferro-magnetic or ferri-magnetic with a coaxial central hole for the beam current. The core imparts a very large inductance to a conducting path that begins on the entire outside circumference of the core at the coaxial feed and wraps around one end to the inside circumference to the opposite end and the housing ground. A high voltage pulse from the coaxial feedline creates a field along a vacuum accelerating gap that drives a beam current (particle beam) through the axis of the core. The vacuum accelerating gap appears to be in parallel with a large inductance. In a typical induction cell, the cell is generally azimuthally symmetric except for a number of coaxial feed lines that supply the accelerating voltage from a pulsed-power unit. The inductive isolation of the voltage persists in time until the core saturates, the inductance reduces to a very low value, and the voltage is shunted to ground. In practice, accelerator cores are driven towards negative saturation after the accelerating pulse to increase the available flux swing. After the application of a reset pulse, the field inside the core will relax to B.sub.r, the remnant field. As the core is subjected to an accelerating pulse, the magnetic domains of the core all align and the permeability of the material falls. The core is then said to be saturated and the field level is B.sub.s.
Unidirectional, direct current, high voltage pulses are used for particle acceleration, e.g., pulsed power systems, rather than high frequency alternating current. Conventional pulsed power systems for induction cells include devices constructed of nested pairs of coaxial transmission lines, so-called "Blumlein" devices, e.g., as shown in FIG. 2. See, U.S. Pat. No. 2,465,840, issued 1948 to A. D. Blumlein, and incorporated here by reference. A step-up transformer or Marx bank slow charging system is connected between an intermediate conductor of the Blumlein and a grounded outer conductor. The output is taken between an inner conductor and the outer conductor which then provides a coaxial drive signal to the induction cell. When the Blumlein is fully charged, there is no net output voltage. But when a switch is closed to ground, a voltage wave is caused to propagate, left to right in FIG. 2, between the inner and outer conductor of the line to the output. This voltage feeds the induction cell with a relatively fast pulse, e.g., on the order of tens of nanoseconds. The switch most often used includes high voltage electrodes separated by an insulating gas, e.g., a spark gap. Conventionally, a third trigger electrode is placed between the main two spark gap electrodes and voltage pulsed to initiate a breakdown. Alternatively, a laser is used to ionize the insulating gas. The breakdown of the gas allows current to flow with a very low resistance. But such systems are repetition-rate limited by the recovery time of the spark gap switch. Higher repetition rates can be realized by blowing the insulating gas through the spark gap switch. Even so, such types of switches are limited to repetition rates that do not exceed several kilohertz.
A 50-MeV advanced test accelerator at Lawrence Livermore National Laboratory was constructed with a pulsed power system that used water-filled Blumleins of beam current for 70 nanoseconds at one Hz for extended periods. It could also provide short power bursts at one kHz by using gas blowers for the spark gaps.
In the early 1980's, free electron lasers were developed which required high average beam power in certain applications, e.g., microwave heating of tokamaks. A magnetic pulse compression power system capable of providing multi-kilohertz operation was developed. Instead of spark gaps, such magnetic pulse compressor systems used saturable magnetic switches, as illustrated in FIG. 3 with a simplified schematic. A capacitance C.sub.1 is slowly charged to approximately twenty-five kV by an external source. When the volt-seconds capacity of the magnetic saturable switch M.sub.1 has been reached, its impedance rapidly collapses and the charge on the capacitor is dumped to ground through the primary of a step-up transformer to produce a still higher voltage across a capacitor C.sub.2. When the volt-seconds capacity of a second magnetic saturable switch M.sub.2 has been reached, capacitor C.sub.2 discharges into a water-filled transmission or pulse-forming line PFL 11. A third magnetic saturable switch M.sub.3 then couples the output of the PFL 11 into a bank of induction cells 13 in parallel. The transfer of energy from one capacitor to the next occurs more rapidly in each succeeding stage if the product of the saturated switch inductance and the storage capacitance drops from one stage to the next. A similar system was used to power the ETA-II accelerator at Lawrence Livermore National Laboratory and is now in fairly wide use. The ETA-II machine produces as many as fifty pulse bursts at rates exceeding three kHz. Each so-called MAG 1-D pulse compressor has been able to drive as many as twenty accelerator cells at approximately 125 kV with a beam current in excess of two kiloamperes (kA).
But such low repetition rates were sorely inadequate by the 1990's. One promising approach to inertial confinement fusion was the use of heavy ion beams to drive the targets. In typical designs, ten GeV uranium ions are needed at tens of kiloamperes for an efficient power plant. Two configurations suitable for heavy ion fusion use induction accelerator technology, e.g., linear induction accelerators and recirculators. Useful recirculators require repetition rates far in excess of those that can be achieved by magnetic pulse compression. The standard approach to providing such beams has been to use induction linacs operated at about ten Hz. But with conventional technology, a linear induction accelerator would need to be about ten kilometers long. Recirculating a beam through small number of induction cells can substantially reduce the cost, but the induction cells would have to be able to operate at pulse repetition rates as high as 100 kHz.
The operational demands imposed on a pulsed power system to properly operate a recirculating induction linac are severe. The accelerating pulse shape and duration are preferably modified as the ions accelerate and the beam is longitudinally compressed. A typical induction linac is capable of producing beams in the kiloampere range with an average accelerating gradient as great as one megavolt/meter.
Vacuum surface flashover or discharge switches initiated by a conventional plasma discharge are conventional. Such switches exhibit low jitter and current rise rates that exceed most all other switches. Surface flashover switches have not been very reliable because such switches must operate very near their voltage breakdown points. Such operation near this threshold voltage, the "self-break electric field", is required for low jitter, e.g., repeatable delays between the time the trigger is received and the time the switch actually closes. A Weibull distribution shows that the reliability of a surface flashover switch operated at 0.90 of the self-break electric field has 0.60 reliability. In contrast, a surface flashover switch operated at 0.60 of the selfbreak electric field is 0.995 reliable.
It has been discovered by the present inventors that the self-break electric field of a vacuum insulator can be lowered significantly if sufficient photons of a given energy are incident on the surface. The self-break electric field can be reduced by 75% with 29 millijoule-cm.sup.-2 248 nanometers fluence onto the surface. The surface flashover appears to occur with very low jitter.